1. Field of the Invention
The present invention relates to a method of measuring particles and an apparatus for the same, which measure the particles in liquid by irradiating a laser beam to the liquid which flows at a fixed flow rate, and detecting light scattered by the particles in the liquid.
2. Related Background Art
In dynamic RAM (DRAM), integration density of integrated circuits (ICs) is shifted from 16M to 64M because of the progress of manufacturing techniques for semiconductor integrated circuits. As the ICs are highly integrated, IC design geometries have been shrinking. In the manufacture of IC, as one of means to improve the yield, it has been known that to improve the purity of chemicals for electronic industries which are used in cleaning wafers and in etching is effective. Evaluation of the purity requires techniques for measuring particles (foreign substances) present in chemicals, which requires the improvement of the accuracy of the particle measuring apparatus and the reliability. Conventionally, such a particle measuring method generally employs the following scattering light detecting method.
That is, the chemical for electronic industries, which is a sample, flows continuously in a flow cell at a fixed flow rate, and a laser beam irradiates to the liquid, which flows in the flow cell, in a direction perpendicular to a direction of the flow of the liquid. When the laser beam is irradiated to the particle present in the chemical, light scattering occurs by the particle. FIG. 1 is a sectional view typically showing a part of the measuring system, and while a particle 2 in sample liquid which passes through a flow cell 1 passes through the irradiation region, light scattering occurs. The light scattering converges in a light detector such as a photomultiplier through an optical system and is converted into a pulse signal. Therefore the particle concentration in chemical is measured by counting the number of pulse signals obtained in the light detector.
However, the particle concentration in the sample is not measured accurately by the conventional scattering light detecting method. That is, because not only are particles but also bubbles present in the sample liquid, the light scattering obtained by irradiating the laser beam include light scattering which is generated by the bubbles. Accordingly, even though the number of pulse signals obtained in accordance with the particles is counted, the pulse signals in accordance with the bubbles are also counted. Therefore, the measurement of the particle concentration may appear too high.
For example, when the chemical is hydrogen peroxide (H.sub.2 O.sub.2), because the chemical is not stable, bubbles (oxygen) are easily generated and this results are not reliable. Since ammonia solution of 30 wt % (weight percentage) has a low boiling point, bubbles are easily generated and therefore accurate results cannot be obtained as well. When such an unstable sample liquid passes through the flow cell the center of which is narrow, its flow rate increases and the pressure is lowered. Therefore, more bubbles are generated. Thus, in the measurement of the particle concentration in a liquid, e.g., hydrogen peroxide, ammonia solution, in which the bubbles are easily generated, it is hard to accurately measure the particles until the bubbles are distinguished therefrom.
Conventionally, in order to solve these problems, a countermeasure of cooling sample liquids, and various other countermeasures have been considered.
First, there is one countermeasure shown in FIG. 2, in which a resin tube 4 having minute blow holes is used. That is, the resin tube 4 is placed in vacuum, and the sample liquid flows in the resin tube 4, whereby the bubbles in the sample liquid are drawn into vacuum through the minute blow holes. After the bubbles are removed, the particle concentration is measured.
Further, there is another countermeasure shown in FIG. 3. That is, the bubbles in the liquid are removed by injecting the sample liquid from a spray nozzle 5, and thereafter the number of particles is counted by a particle counter PCM.
It is preferable that a general control level of particles in the electronic industrial chemical is below 10.about.100 pieces/ml. However, in any cases of the above-described countermeasures, it is hard to remove bubbles to a level which does not have effects on the particle measurement.